The application of clear top coats over basecoat paint layers is becoming increasingly popular in the automotive industry. In such a color-plus-clear or basecoat/clearcoat system the pigmented basecoat is sprayed on the automotive panel and is overcoated with a reactive clear composition which crosslinks and hardens after being sprayed and dried. For example, the recent patent to Simpson et al, U.S. Pat. No. 4,681,811, describes a transparent topcoat formed by spraying over the paint layer a liquid crosslinkable composition comprising a polyepoxide and a polyacid curing agent. According to the patent, this provides a composite with outstanding gloss and distinctness of image. Simpson et al give no suggestion that any problems result from mixing the components of the clearcoat composition well in advance of spraying the mixture.
Another kind of clear top coat composition for application by spraying is described by Ambrose et al, U.S. Pat. No. 4,699,814. This contains a low molecular weight epoxy-functional polyurethane obtained by reacting an isocyanate with a hydroxy polyepoxide. A polyacid curing agent is used. Again no problem is indicated with mixing the components well before the mixture is applied.
Recently a new process for applying paints and clearcoats to automobile bodies and other three dimensional articles which yields finishes of outstanding quality has been described in the Reafler patent application Ser. No. 116,426. The new process involves coating one side of a stretchable polymeric film by laminar flow with a pigmented basecoat, thereafter applying a clearcoat over the basecoat and applying an adhesive layer to the other side of the film, which then is adhered by thermoforming to automobile body panels or other substrates, for example, in the manner described in European Patent Application 0251546 published Jan. 7, 1988. The coating technique used for optimum quality involves the continuous laminar flow coating of a moving web of the plastic film by contact with a horizontally extending bead of the liquid coating composition. The latter is extruded from a narrow horizontal slot of an extrusion coating hopper.
Extrusion coating hoppers have been used previously in precision coating of photographic goods, as disclosed, for example, in the patent to Miller et al, U.S. Pat. No. 3,206,323. They can produce coatings having exceptional smoothness, gloss and other good qualities. They are, however, more sensitive to problems with the feed composition than is the conventional spray painting method of applying clear coats to automobile bodies. Thus, it has been found that reactive clearcoat compositions begin to increase in viscosity soon after the components are mixed. If the mixed clearcoat composition is held in a supply vessel for an excessive length of time before being fed to the extrusion hopper the viscosity of the feed composition may increase during the coating operation. When this occurs, the extrusion hopper may produce coatings of less than optimum quality.
It has also been found that after the components of the clearcoat have been mixed in the supply vessel, heterogeneous particles or slugs of polymer begin to form. These tend to stick to the metal in and around the coating hopper exit slot and cause line defects in the coatings. Evidently neither of these phenomena cause problems when reactive clearcoat compositions are sprayed, possibly because the high shear of the spray nozzles lowers the viscosity of the coating composition.
In accordance with the present invention it has been found that potential problems in the coating of batch mixed reactive clearcoats on paint-coated polymeric films through the narrow slot of an extrusion coating hopper can be reduced or avoided by mixing continuous streams of the components of the clearcoat shortly before the mixture is coated on the moving plastic film web.